1. Field of the Invention
This invention relates to a high-frequency oscillator circuit which outputs fundamental and second harmonic waves, which can be used in motor vehicle telephone sets, cordless telephone sets, television tuner circuits, and the like, and which can be suitably arranged in the form of an integrated circuit.
2. Description of the Related Art
In the field of mobile communication, where a tendency to reduce the size of communication apparatus is shown, there is a need to provide a high-frequency oscillator circuit in the form of an integrated circuit. Also, it is important that a second harmonic can be efficiently extracted from such a high-frequency oscillator circuit.
FIG. 3 is a circuit diagram showing the construction of a conventional high-frequency oscillator circuit which outputs a fundamental wave and the second harmonic of the fundamental wave. The high-frequency oscillator circuit shown in FIG. 3 has a transistor 1 for high-frequency oscillation and amplification, a fundamental wave resonance circuit 2, formed of an inductor 3, a capacitor 4, and a variable capacitor diode 5 for tuning, and a second harmonic resonance circuit 6 formed of an inductor 7 and a capacitor 8.
A capacitor 9 serves to check a direct current. A frequency control voltage V.sub.CTL outputted from a frequency control circuit (not shown) is supplied to a frequency control terminal 11 which connects to the fundamental wave resonance circuit 2 through a resistor 10. The frequency of the fundamental wave oscillated in this high-frequency oscillator circuit is changed through the frequency control voltage V.sub.CTL. A power supply voltage V.sub.CC is supplied to a power supply terminal 12. A bypass capacitor 13 is inserted between the power supply terminal 12 and ground.
Further, there are provided feedback capacitors 14 and 15, a constant-current circuit 16 capable of outputting a constant current, and resistors 17 and 18. The circuit elements 16 to 18 serve to bias the transistor 1 so that the transistor 1 operates at a predetermined operating point.
The collector of the transistor 1 is connected through a signal extraction capacitor 19 to a second harmonic output terminal 20 through which the second harmonic is outputted. The emitter of the transistor 1 is connected through a signal extraction capacitor 21 to a fundamental wave output terminal 22 through which the fundamental wave is outputted.
In the above-described high-frequency oscillator circuit, it is necessary to tune the second harmonic resonance circuit 6 in order to efficiently extract the second harmonic. However, the signal characteristics of the second harmonic resonance circuit 6 change through a wide frequency range, and it is therefore difficult to extract the second harmonic at a constant level through a wide frequency range. At frequencies above the UHF band, in particular, the power loss in the second harmonic resonance circuit 6 is large and the output power cannot be efficiently extracted.
There is also the problem of a reduction in carrier to noise (C/N) ratio with respect to both the fundamental and harmonic waves, because the second harmonic resonance circuit 6 influences the oscillation of the fundamental wave.
Moreover, for tuning the second harmonic resonance circuit 6, adjustment of inductor 7 and capacitor 8 is required. It is therefore difficult to design the oscillator circuit as an integrated circuit.